Internal combustion engines are typically designed to operate at optimum efficiency and performance by the selection and design of a desired compression ratio, i.e., a ratio of maximum to minimum cylinder volume, during operating conditions. However, circumstances exist in which it may be desired to change a compression ratio, perhaps dynamically. For example, a compression ratio which may be desired under normal engine run conditions may not be effective during engine start conditions.
Various methods have been devised for varying the compression ratio of an engine. For example, in U.S. Pat. No. 5,682,854, Ozawa discloses a variable compression ratio engine which changes the compression ratio by alternating between a normal operating mode to a Miller cycle operating mode. The compression ratio can thus be changed from one range, e.g., 15-17, to a second range, e.g., 11-13. Ozawa's disclosed invention, however, cannot vary the compression ratio over the entire range of possibilities, for example from 11 to 17, but rather is intended to “switch” the compression ratio from one value to another. It has been found, moreover, that operating in Miller cycle, i.e., varying actuation of intake valves to change compression ratio, can in itself only achieve a limited variance of compression ratio. For example, varying compression ratio by varying valve timing may only effectively change compression ratio over a range of about 1-2. That is, by varying intake valves, it is typically feasible to vary compression ratio from exemplary values of 11:1 to 13:1.
Another common technique for varying compression ratio is to alter the volume of a cylinder by some method. For example, U.S. Pat. No. 4,144,851 to Prosen, U.S. Pat. No. 4,873,947 to Ryan, III et al., and U.S. Pat. No. 5,329,893 to Drangel et al. all indicate engines which can vary the compression ratio by changing the volume of the cylinders. However, in these cases, the methods employed require extensive modifications of the engines themselves, including the engine block. These modifications are very costly and add a great deal of complexity to the engines. Furthermore, they require extensive redesign of auxiliary systems, such as air intake and exhaust systems, to allow these systems to move in cooperation with the changing engine configurations.
Another method for varying compression ratio is to incorporate auxiliary chambers which vary the effective volume of the cylinders. An example of this is found in U.S. Pat. No. 6,450,154 to Choi. A secondary chamber having a secondary piston is used to change the compression ratio of a primary chamber. The apparatus of Choi, however, alters the geometry of the cylinder, i.e., the combustion chamber, to achieve a variable compression ratio. In many instances, for example when a uniform fuel distribution in the cylinder is desired, altering the geometry of the cylinder is detrimental to operation of the engine.
The present invention is directed to overcoming one or more of the problems as set forth above.